Bioenergy Fact Sheet Last Update 2020
Torrefied pellets
DEFINITION AND PROPERTIES
Torrefied pellets, also called black pellets, are a densified biofuel, made from thermally treated solid biomass, with or without additives. They usually have a cylindrical form, a length of 5 to 40mm, a diameter up to 25mm and broken ends. The common raw material of torrefied pellets is sawdust, but every feedstock which can be pelletized can be torrefied. Properties of torrefied pellets (TW2a) Alternative feedstocks include bark, miscanthus or Bulk density (kg/m³) ≥ 650 agricultural residues. There is a European standard (DIN Moisture (w-%) ≤ 8 EN ISO 17225-8:2016: Solid biofuels – Fuel Lower heating value (MJ/kg) ≥ 20.2 specifications and classes – Part 8: Graded thermally Mechanical durability (w-%) ≥ 96 treated and densified biomass fuels) which defines Ash content (w-%, dry) ≤ 3 specifications of torrefied pellets. Table 1 shows properties of torrefied pellets in medium quality Table 1: Properties according ISO/TS 17225-8:2016 (TW2a). Compared to white pellets, torrefied pellets have lower moisture content and a higher heating value, but higher ash content. Properties vary depending on the feedstock, especially when using low-quality feedstocks. Research on improving the properties of torrefied pellets from different feedstocks is ongoing.
THE TORREFACTION PROCESS
Torrefaction is a thermochemical process (Figure 1). In the first step, biomass with a humidity of about 20 to 50% is dried. It is possible to use gases from the torrefaction process, in order to increase efficiency. Subsequently, the biomass is torrefied, which means it is heated up to between 200 to 350°C, under atmospheric pressure, without oxygen, for 30 to 60 minutes. The high temperature leads to decomposition of biomass components. Low-energy components are transferred to the gas phase. The water is expelled first and then hemicellulose and partly also lignin decompose. The partly decomposed (torrefied) biomass is grinded and afterwards pelletized. It is also possible to interchange the torrefaction and pelletization steps. In that case ready-made pellets are subsequently torrefied. The whole production can be stand-alone or integrated in e.g. a large power plant boiler, a sawmill, or a bio- oil production (pyrolysis).
Figure 1: Scheme of torrefaction process Bioenergy Fact Sheet Last Update 2020
ADVANTAGES AND APPLICATIONS
Torrefaction improves the combustion properties of the pellets. Torrefied pellets have higher energy density and heating value compared to white pellets. Additionally, problems during storage (e.g. soaking or outgassing) are mitigated, because of increased resistance to water absorption and decreased biological activities. This enables outdoor storage of torrefied pellets. Furthermore, there is less energy needed for grinding in comparison to white pellets. Therefore torrefied pellets are suitable for co-combustion in coal power plants. The application of torrefied pellets is mainly at industrial scale. The increased energy density also reduces transport costs.
At present, torrefied pellets are mainly produced for large-scale co-firing in pulverized coal power plants. Co-firing of torrefied pellets decreases share and amount of fossil fuels, reduces CO2 emissions and is therefore fully in line with green energy support mechanisms. Torrefied pellets can also fully substitute for coal in new-built facilities.
EXAMPLES OF TORREFIED PELLET PLANTS
Operator: Baltania Tallin, Estonia
Commercial plant (under construction) Location: Vägari, Estonia Funding: € 25 million Feedstock: Woody biomass (bark, harvesting residues, firewood, by-product chips, pulpwood) Capacity: 157.000 tons/a Technology: CEG, The Netherlands
Operator: Futerra Fuels Valongo, Portugal
Commercial plant (under construction) Location: Valongo, Portugal Investment: € 38 million Feedstock: Eucalyptus, biomass residues Capacity: 120.000 tons/a torrefied pellets, 55,000 tons/a white pellets Technology: Yilkins, The Netherlands
Bioenergy Fact Sheet Last Update 2020
SOURCES
. https://www.vtt.fi/inf/pdf/technology/2014/T163.pdf . http://task40.ieabioenergy.com/wp- content/uploads/2013/09/IEA_Bioenergy_T32_Torrefaction_review.pdf
FURTHER INFORMATION
. https://baltania.ee/ . https://ibtc.bioenergyeurope.org/ceg-delivers-200-tonnes-of-ceg-biocoal-for-a-european- customer-trial/ . https://futerrafuels.com/en/our-production-plant . https://yilkins.com/